not that it was an engine redo.. but Everquest went from i want to say DX8 to 9... or 7 to 8 .. i dont remember which arround the time that planes of power came out..

and it cause HUGE issues... people were falling into the ground.. you couldnt get to places you could before... some places were very hard to reach after the change... and it left a sour taste in many peoples mouths ...

imo ... its just not a good idea to do .. to much time has already been spent to make things work right the way they do.